A semiconductor diode is a two-electrode semiconductor device, having an anode and a cathode, which has marked unidirectional electrical characteristics. A junction diode is a semiconductor diode whose asymmetrical voltage-ampere characteristics are manifested as a result of a PN junction formed at the transition between N-type and P-type regions within the semiconductor wafer. This junction may be either diffused, grown or alloyed.
A high power diode generally requires that one of the regions, usually the anode region, have a low impurity concentration, e.g., 1 .times. 10.sup.14 to 1 .times. 10.sup.16 atoms per cm.sup.3. This enables the device to withstand a high reverse blocking voltage without breakdown or punch-through by permitting a wide space charge region. A difficulty with such devices has been the long reverse recovery time upon breakover into the conduction mode. That is, the time needed for the device to reestablish the blocking mode upon breakdown or punch-through. Such recovery time is primarily dependent upon the recombination time of the minority carriers in the highly resistive region, which as previously stated is usually the anode region.
It has been proposed to reduce the reverse recovery time of diodes by irradiation, see application Ser. No. 339,242 filed Mar. 8, 1973, now U.S. Pat. No. 3,809,582, and assigned to the same assignee as the present invention. The irradiation damages the atomic lattice causing the formation within the atomic lattice of large numbers of defects corresponding to energy levels within the forbidden energy gap between the valence and conduction energy bands of the semiconductor. These energy states increase the recombination rate of the minority carriers without correspondingly increasing the carrier generation rate. Thus, the reverse recovery time of the diode can be substantially reduced without correspondingly increasing the forward voltage drop and other electrical characteristics.
The difficulty with such radiation techniques has been the erratic effect of irradiation upon the electrical characteristics. That is, the effect of a given dosage of radiation from a radiation source on the electrical characteristics cannot be predicted from device to device. Specifically, it has been known that minority carrier lifetime before and after irradiation can be given by the relationship:
1/.tau. = 1/.tau..sub.o + K.sub.L .phi. PA1 .tau. is the post-irradiation minority carrier lifetime; PA1 .tau..sub.o is the pre-irradiation minority carrier lifetime; PA1 .phi. is the radiation dosage; and PA1 K.sub.l is the minority carrier lifetime damage factor. PA1 1/.tau. = 1/.tau. .sub.o + K.phi. PA1 .tau. is the desired nominal minority carrier lifetime; PA1 .tau..sub.o is the measured nominal minority carrier lifetime; PA1 K is the minority carrier lifetime damage factor; and PA1 .phi. is the irradiation dosage.
where
However, the damage factor K.sub.L varies widely from device to device. The damage factor has been known to vary with the type and concentration of impurities, temperature of application, and type and intensity of radiation, as well as the bulk of the semiconductor crystal.
Irradiation has not therefore been considered useful for fine correction of the electrical characteristics of devices. Rather, it was considered useful to substantially change certain electrical characteristics without corresponding changes in other electrical characteristics, where the devices were subsequently tested and classified or subsequently annealed. See also patent application Ser. No. 324,718, filed Jan. 18, 1973, now U.S. Pat. No. 3,881,963, Ser. No. 283,684, filed Aug. 25, 1972, now U.S. Pat. No. 3,872,493, Ser. No. 283,685, filed Aug. 25, 1972, now U.S. Pat. No. 3,840,887, Ser. No. 285,165, filed Aug. 31, 1972, now abandoned, Ser. No. 343,070, filed Mar. 30, 1973, now U.S. Pat. No. 3,877,997, Ser. No. 354,620, filed Apr. 25, 1973, now abandoned, and Ser. No. 337,967, filed Mar. 5, 1973, now U.S. Pat. No. 3,881,968, all of which are assigned to the same assignee as the present invention.